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Scientists have discovered a shard of a planet that survived the death of its star and TESS has found the first direct evidence of an exocomet.

Host: Hank Green

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[ ♪ Intro ].

When stars like the Sun reach the end of their lives, they balloon into red giants and swallow up any planets in their way. In our solar system, that will be the end of Mercury, Venus, and Earth.

They then shrink to a dying remnant called a white dwarf, which is sometimes surrounded by a disk of, like, ex-planetary dust. But in an extrasolar system 410 light-years away, scientists discovered a shard of a planet that survived the death of its star, according to a study published last week in the journal Science. They discovered the object in orbit around a white dwarf.

In the star's spectrum, they saw a repeated pattern of light dimming and brightening. This told astronomers that something was moving through the star's disk of dust and creating a cloud of gas. That gas emitted a little extra light that was noticeable each time it went around the star.

And whatever was causing it seemed to be somewhere between a few kilometers and a few hundred kilometers wide. This makes it only the second solid body ever discovered around a white dwarf. Now, the object's circumstances are pretty grim.

Not only is it a lone survivor circling a dead star, but it's sitting in a disk of debris made of the remains of destroyed planets. But the fact that it exists at all is pretty incredible. It's way closer than astronomers expected to find any planetary survivors.

It's so close, it's actually thought to be orbiting within the star's original radius. Meaning, if you replaced the white dwarf with our Sun, it would be orbiting inside of the Sun. Yeah.

So this thing orbits very quickly. Fast enough that it rings in a new year every two hours. At this distance, the star's gravity is so intense, most planets or asteroids would be ripped apart.

But somehow this fragment has held up. Scientists suspect that it's because it's made mostly of heavy metals like iron and nickel. And such a strong chunk of metal could have survived the destruction of its planetary system, the researchers say. “Metal” is also the word I would use to describe this planetary fragment's, like, whole thing.

The researchers' best guess is that it's the metal core of a larger planet that used to orbit farther out, before the death of its star caused all the planetary mayhem. And this fragment of a past planet can tell us more about what kinds of planets make up extrasolar systems. It can also offer us a hint at our solar system's future, and what traces might remain after our Sun dies.

This planetary remnant isn't the only interesting thing that has turned up recently in a disk of debris, either. In a preprint of a paper accepted for publication in the journal Astronomy & Astrophysics, researchers announced that they'd discovered a comet around the young star Beta Pictoris. That's right, an exocomet.

They found it in data from TESS, the exoplanet-hunting satellite that launched last year as a successor to Kepler. And the fact it's already got an exocomet discovery under its belt is pretty promising. There's been pretty strong evidence for a while that exocomets are out there.

Astronomers studying the emission spectra of stars saw that sometimes certain wavelengths of light were getting absorbed for a few hours at a time. So they could infer that something comet-sized was passing in front of these stars. One 2014 study even suggested that there were thousands of exocomets around Beta Pictoris alone.

But it was hard to be certain about any of this. The best evidence would be in the form of a light curve: a subtle dip in brightness as one of these comets passed in front of its star. It wouldn't look like the dip that comes from a planet.

Planets are basically symmetrical, so the light dims as the planet passes in front and then brightens when it leaves. But comets have tails. So the first half of an exocomet's light curve should look like a planet's, a sharp drop in brightness.

But then the drop should reverse more gradually, as light starts to peek through the tail. Astronomers predicted 20 years ago that a comet would have this kind of light curve, but our observations are just starting to catch up. In its 9 years of operation, the Kepler telescope found three potential instances of exocomets.

But Kepler paid more attention to older stars, which we think have fewer comets, as the chaos of planetary formation calms down a bit. TESS looks at all kinds of stars, including young ones, which are still surrounded with some building blocks that formed planetary systems. And the fact that it's already found one exocomet gives us a lot of reasons to get excited about the science that's ahead.

Now that they know what TESS is capable of, scientists will be on the lookout for more signals like this. With more data, they can start to see what kinds of stars typically have lots of comets, or don't. And that can tell us about the role comets play in the formation of planetary systems, and how common they actually are.

So, whether it's in the debris surrounding a young star or the rubble swirling around a dead one, there's a lot of galactic archaeology to do in the years ahead. Thanks for watching this episode of SciShow Space News, which is brought to you with the help of our patrons on Patreon. If you like what we do here and want to help us out, check out to learn how. [ ♪ Outro ].